We have a unique in vitro system for study of the factors controlling leukemic transformation of bone marrow hemopoietic stem cells. Long-term corticosteroid-supplemented marrow cultures of C57BL/Ks/J mice (a strain showing a low incidence of thymic lymphoma in vivo) demonstrate a 100% frequency of generation of continuous promyelocytic leukemia cell lines by 12 weeks growth in vitro. Induction of an ecotropic endogenous retrovirus uniformly precedes leukemogenesis in these cultures. Leukemia cell lines are of granulocytic phenotype by morphology and biochemical markers, and induce granulocytic leukemia following inoculation into newborn or adult syngeneic mice. In contrast, none of over 400 NIH/Swiss mouse marrow cultures similarly maintained in excess of 20 weeks released detectable virus or generated leukemia cell lines. Study of the cellular and viral requirements for leukemia in these parent, F1-hybrid, F2, and F1 backcross strains is proposed. Methods include: establishment of C57BL/Ks/J embryo cell lines for iododeoxyuridine (IudR) induction and characterization of endogenous viruses; generation of (NIH/S x C57BL/Ks/J) F1 hybrid, F2, and backcross mouse marrow for in vitro culture; long-term corticosteroid-supplemented mouse marrow cultures; in vitro assays for granulocyte macrophage progenitor cells (CFUc), B-lymphocyte colonies (CFUL), burst-forming unit-erythroid (BFUe), spleen colony assay for pluripotent hemopoietic stem cells (CFUs); suspension culture of leukemia lines; and tumorigenicity studies in newborn and adult mice.